The goal of this project is to get a better understanding of outer hair cell (OHC) electromotility by analyzing the 3D architecture of the molecular complexes in the OHC lateral wall using electron tomography (ET). Like other hair cells, the OHC generates a receptor potential following hair bundle stimulation. However, in the OHC, this receptor potential results in cell length changes. This motility is unique to the OHC, and is localized to its lateral wall. Because the conversion of transmembrane potential to mechanical energy occurs in the cell membrane and is associated with the transmembrane protein Prestin, the architecture of the underlying cytoskeleton (cortical lattice) and membranous structures of the subsurface cisternae (SSC) are critical to understanding the transfer of this energy to the OHC and supporting cells of the organ of Corti. To date, most knowledge of lateral wall structure has been limited to extrapolation from 2D studies of samples, further complicated by preservation artifacts depending on the sample preparation method used. Structural features of the OHC lateral wall, such as pillar protein and actin filament orientation, are inherently three-dimensional in nature. Also, computational models of OHC motility and electrophysiology depend upon faithful depiction of 3D architecture of molecular complexes and boundaries of intracellular compartments. Thus, we propose to use ET to map the 3D structure of the molecular machines in the lateral wall, providing the first 3D map of the relationships of the cell membrane, cortical lattice, and SSC in an intact cell. Specimens will be prepared using high pressure freezing and freeze substitution (HPF/FS), ensuring the highest-fidelity specimens available and eliminating artifacts present in previous studies. The specific aims of this proposal are: 1) to determine the 3D relationships of molecular and membranous components of the lateral wall, 2) to characterize a novel structure in the lumen of the SSC, and 3) to apply new correlative alignment and averaging techniques to refine the structure of lateral wall components defined by ET. Outer hair cells are required for mammalian hearing, and their loss results in deafness. This study will explain the structure underlying the motility of the OHC, vital to our understanding of OHC operation and function in the cochlea. The methods pioneered in this study will also establish a foundation on which we can study the structural correlates of hearing related pathologies, such as actin and myosin disorders that lead to deafness.